This invention relates to a marine propulsion system, and more particularly to a trimmable stern drive system.
In a stern drive marine propulsion system, it has been found advantageous to employ a system for automatically adjusting the trim position of the stern drive unit in response to one or more preselected operating conditions, such as engine speed or boat attitude. Such a system automatically increases boat speed during high speed operation by adjusting the trim angle so as to reduce boat drag, and eliminates the need for the boat operator to manually adjust trim position. Additionally, it has been found that an increase in engine power output can be achieved by routing engine exhaust through the boat transom to an above-water exhaust outlet, as opposed to routing the exhaust through the drive unit and discharging through a below-water exhaust outlet, such as through the propeller hub. However, discharge of exhaust through an above-water outlet results in an increase in the level of noise produced during boat operation.
The object of the present invention is to combine the increase in boat speed resulting from employment of an automatic trimming system, with the increase in power which can be achieved by selectively routing exhaust to an above-water discharge instead of to a below-water discharge. In accordance with the invention, a marine propulsion system includes an engine interconnected with an exhaust discharge system, which includes an above-water exhaust discharge outlet and a below-water exhaust discharge outlet. Exhaust control means is provided for controlling discharge of exhaust through the above-water outlet and through the below-water outlet. A trimmable drive unit is drivingly interconnected with the engine, and trimming means is provided for trimming the drive unit. The invention discloses means interconnecting the exhaust control means and the trimming means so that the exhaust control means is responsive to actuation of the trimming means. In a preferred embodiment, the trimming means is operable in response to one or more preselected boat operating conditions, such as engine speed or boat attitude. That is, for example, when engine speed reaches a certain predetermined level, the trim position of the drive unit is automatically adjusted so as to provide an increase in boat speed by reducing drag. When engine speed falls below the predetermined level, the trimming means trims the drive unit in. The automatic trimming means preferably includes control means for outputting a signal for controlling the trim position of the drive unit, and the means interconnecting the trimming means and the exhaust control means is preferably responsive to the output signal provided by the control means.